Can my novel be written in first person — or alternate between my two leads like romance novels do?

Yes. For fiction books, the Voice step of the wizard (and Book details on an existing book) offers a Narration setting: Third person (the default), First person (one "I" voice for the whole book), or First person alternating — the romance convention where the two leads take turns chapter by chapter. The alternation is enforced for the WHOLE book: odd chapters follow your first lead, even chapters the second (define both as characters in the book's Codex; a chapter's own POV setting, from the small POV button on its row, always wins). Every chapter is generated with an explicit instruction never to drift back into third person — so the pattern doesn't fade out after a few chapters.